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  • Open Access

Cost-effectiveness of cognitive behavioural therapy for treatment-resistant depression: challenges and solutions conducting an economic evaluation of the long-term follow-up of the cobalt trial

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Trials201516 (Suppl 2) :P24

  • Published:


  • Economic Evaluation
  • Cognitive Behavioural Therapy
  • Unit Cost
  • Multiple Imputation
  • Variable Interval


Most trial-based economic evaluations are restricted to the follow-up period of the trial. We present challenges faced conducting an economic evaluation of the long-term (LT) follow-up of a successful trial and describe solutions, which could inform similar studies.


An economic evaluation was conducted alongside the LT follow-up of the CoBalT trial assessing the effectiveness and cost-effectiveness of cognitive behavioural therapy (CBT) for patients with treatment resistant depression.

Challenges included: follow-up at a variable interval after randomisation date; resource use data from a questionnaire smaller in scope than the trial and available for a limited period; inconsistent availability of unit costs and missing data due to loss to follow-up. Challenges were addressed by: combining questionnaire data with information from the trial to estimate average annual values of costs over the whole follow-up period; collecting detailed health care resource use over the whole period for a sample using practice notes; using a mix of inflation-adjusted and updated unit costs; and multiple imputation to estimate missing data.


The LT complete case analysis included 214 of the original 469 participants. Mean annual incremental cost to the NHS was £281; QALY gain was 0.052. The incremental cost-effectiveness ratio was £5,374. At a threshold willingness-to-pay of £20,000, this represents a 92% probability of cost-effectiveness. Results remained robust in sensitivity analyses.


Despite methodological challenges, using all available information and a variety of modelling and imputation techniques, we were able to estimate annualised costs and effects of a CBT intervention over the long term.

Authors’ Affiliations

University of Bristol, Bristol, UK
Bristol Randomised Trials Collaboration, Bristol, UK


© Garfield et al. 2015

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver ( applies to the data made available in this article, unless otherwise stated.


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